It seems to be an upstream bug which prevents linking when XML backend
is not selected:
../libiio.so.0.21: undefined reference to `encode_xml_ndup'

Until this is clarified with upstream, prevent OpenWrt users from
deselecting the XML backend. This increases the footprint of this
package, but renders it usable again.

Signed-off-by: Michael Heimpold <mhei@heimpold.de>